"558 Dr. H. Lester on the Variation of 



density in the film is established consistent with the new 

 velocity. Similarly, upon suddenly decreasing the potential 

 from 400 to 40 volts, there is a decreased current during the 

 time that the film " fills up " to an equilibrium density. The 

 fact that equilibrium was established more quickly at high 

 than at low potentials is consistent with the idea that the 

 speed of the ion in the film depends largely upon the applied 

 potential. 



The above discussion has considered only positive currents. 

 However, the same film-conditions would produce similar 

 effects for both the positive and the negative emissions. In 

 the case of the negative currents most of the carriers are 

 electrons, since in only a few cases have negative ionic 

 carriers been detected. 



The double-range phenomena observed with CaO and BaO 

 cathodes have been observed by Richardson and Cooke forjan 

 osmium cathode and by Langmuir for a tungsten cathode. 

 The instability in current was no doubt caused by an insta- 

 bility in the surface-film. Probably the ionized material 

 in the film becomes unstable, so that chemical combination 

 occurs forming new compounds with characteristic emitting 

 properties. The suddenness of the change from one compound 

 to another was surprising. The large changes in current 

 that occurred illustrate the great importance of surface-films 

 in modifying thermionic emission. 



Summary. 



It has been shown that there is an initial growth in the 

 value of lack of saturation, and that there is a peculiar over- 

 shooting effect. These facts, together with more generally 

 known behaviour of initial thermionic currents, indicate 

 that the observed peculiarities of initial thermionic emission 

 may be explained as due to the influence of ionized surface- 

 films. 



In conclusion, I would like to express my indebtedness to 

 Professor 0. W. Richardson, under whose direction the work 

 was begun, and to Professor H. L. Cooke and Professor 

 E. P. Adams, who have offered valuable criticism and 

 advice. 



